Moravian astronomer, born at Mederizenhi in Moravia, 20 Aug., 1719, died at Heidelberg, 16 April, 1783. He entered the Society of Jesus at Mannheim on 26 Sept., 1745, and after completing his studies taught the humanities for some tirne at Aschaffenburg. He likewise cultivated his taste for mathematics, and later was appointed professor of mathematics and physics in the University of Heidelberg. In 1755 he was invited by the Elector Palatine Charles Theodore to construct and take charge of astronomical observatory at Mannheim. Here as well as at Schwetzingen, where he had also built an observatory, he carried on his observations which led to numerous memoirs, some of which were published in the "Philosophical Transactions" of London. One of his observations, recorded in the "Tables d'aberration et de mutation" (Mannheim, 1778) of his assistant Mesge, gave rise to much discussion. He claimed to have discovered that many of the more conspicuous stars in the southern heavens were surrounded by smaller stars, which he regarded as satellites. His contemporaries, including Herschel and Schröter, who were provided with much more powerful telescopes, failed to verify his observations. Mayer, however, defended their reality and replied to one of his critics, the well-known astronomer Father Höll, in a work entitled "Gründliche Vertheidigung neuer Beubachtungen von Fixstern-trabanten welche zu Mannheim auf der kurfürstl. Sternwarte endecket wordern sind", (Mannheim, 1778). In the following year he published a Latin work on the same subject. The observations, which were made in good faith, were evidently due to an optical illusion. Mayer spent some time at Paris in the interests of his science, and visited Germany in company with Cassini. Upon the invitation of Empress Catherine of Russia, he went to St. Petersburg to observe the transit of Venus in 1769. He was a member of numerous learned societies, including those of Mannheim, Munich, London, Bologna Göttingen, and Philadelphia. He published a number of memoirs, among which may be mentioned "Basis Palatina" (Mannheim, 1763), "Expositio de transitu Veneris" (St. Petersburg, 1769), "Pantometrum Pacechianum, seu instrumentum novum pro elicienda ex una statione distantia loci inaccessi" (Mannheim, 1762); "Nouvelle méthode pour lever en peu de temps et à peu de frais une carte générale et exacte de toute la Russie" (St. Petersburg, 1770); "Observations de la Comète de 1781" in the "Acts Acad. Petropolit." (1782), etc.